The present invention relates to a photocatalyst that can produce effects when exposed to visible light.
Conventionally, various materials such as, for example, TiO2 (titanium dioxide), CdS (cadmium sulfide), WO3 (tungsten trioxide), and ZnO (zinc oxide) are known as materials for producing photocatalytic effects. These photocatalytic materials are semiconductors which absorb light to produce electrons and holes, and cause various chemical reactions and disinfection effects. Currently, the only material put in practice is TiO2, because TiO2 is superior in consideration of toxicity and stability with respect to exposure to water, acids, and bases.
However, because of the band gap value of TiO2 (Eg=3.2 eV in an anatase crystal), the operational light of the TiO2 photocatalyst is limited to ultraviolet light having a wavelength of less than 380 nm. In order to allow satisfactory operation under sunlight, indoors, or in a vehicle, and to improvement catalytic activity when light of weak intensity is irradiated, there is strong demand for development of a material which can realize catalytic activity when irradiated by visible light having a wavelength longer than or equal to 380 nm.
For example, Japanese Patent Laid-Open Publication No. Hei 9-262482 discloses modification of material through ion implantation of a metal element such as, for example, Cr (chromium), and V (vanadium) to an anatase TiO2 which has a high catalytic activity, in order to shift the light absorption edge of TiO2 towards a longer wavelength and to thereby enable operation of a TiO2 catalyst under visible light. Although doping of Cr, V, or the like has been reported since the early 1970""s, none of the early reports disclose that operation by visible light is enabled. In Japanese Patent Laid-Open Publication No. Hei 9-262482, operation by visible light are enabled by using a special doping method, ion implantation, for Cr, V, or the like.
In the above conventional art, operability under visible light of a TiO2 photocatalyst is enabled through ion implantation of a metal element to TiO2. However, ion implantation of a metal element is likely to require large and expensive apparatus. To this end, there is a demand for synthesizing the TiO2 photocatalyst through other methods, such as, for example, synthesis in solution or sputtering. However, photocatalyst created through these methods cannot operate by visible light. It is considered that this is because aggregation of the dopant, Cr, occurs or because oxides such as Cr2O3 are formed during the crystallization processes. As described, in the conventional art, there has been a problem that, in order to enable operation of TiO2 by visible light using a metal element, ion implantation of the metal element is required.
One object of the present invention is to realize a TiO2 photocatalyst capable of operating in the visible light in addition to the ultraviolet range by using a novel material and without using costly production methods such as ion implantation.
According to a first aspect of the present invention, there is provided a photocatalyst comprising, as an inner material, a titanium compound (Tixe2x80x94Oxe2x80x94N or Tixe2x80x94Oxe2x80x94S) in which a nitrogen atom (N) or a sulfur atom (S) substitutes for a portion of the oxygen site of crystals of titanium oxide (for example, TiO2), is doped at an interstitial site of the crystal lattices of titanium oxide, or is placed at the grain boundary of polycrystalline assembly of titanium oxide crystals, and wherein a charge separation material is partially supported on the surface of the titanium compound.
Tixe2x80x94Oxe2x80x94N or Tixe2x80x94Oxe2x80x94S are titanium compounds obtained by introducing nitrogen or sulfur to titanium oxide crystals and have an active photocatalytic function not only when exposed to light in the ultraviolet range, but also under light in the visible range. Therefore, the photocatalytic function similar to that in TiO2 can be obtained with visible light as the operational light.
Moreover, a charge separation material can be partially supported on the surface of Tixe2x80x94Oxe2x80x94N or Tixe2x80x94Oxe2x80x94S. As the charge separation material, for example, at least one of Pt, Pd, Ni, RuOx (for example, RuO2), NiOx (for example, NiO), SnOx (for example, SnO2), AlxOy (for example, Al2O3), ZnOx (for example, ZnO), and SiOx (for example, SiO2) may be selected. Such a charge separation material acts as a promoter and facilitates separation of charges produced as a result of irradiation of light. That is, a metal element such as Pt, Pd, and Ni selectively captures electrons and an oxide such as RuOx (for example, RuO2), NiOx (for example, NiO), SnOx (for example, SnO2), AlxOy (for example, Al2O3), ZnOx (for example, ZnO), and SiOx (for example, SiO2) selectively captures holes. Therefore, by partially supporting these materials on the surface of the photocatalytic material, the probability of recombination of electrons and holes produced by the photocatalytic reaction is reduced and, thus, reduction in activity caused by the recombination of electrons and holes can be prevented.
It is preferable that the ratio, X %, of number of atoms of N in Tixe2x80x94Oxe2x80x94N be 0 less than X less than 13. A similar ratio is preferable for the S in Tixe2x80x94Oxe2x80x94S. It is also preferable that, when the metal element or oxide is assumed to be uniformly supported, the amount of the metal element or oxide on the surface which acts as a promoter corresponds to a thickness of 0.1 angstrom (xc3x85) to 10 xc3x85. In the case of SiOx, it is preferable that the corresponding amount be 10 xc3x85 to 500 xc3x85. In reality, these promoters on the surface forms an island-like structure and may not be present entirely over the Tixe2x80x94Oxe2x80x94N or Tixe2x80x94Oxe2x80x94S surface.
According to another aspect of the present invention, it is preferable that the Tixe2x80x94Oxe2x80x94N or Tixe2x80x94Oxe2x80x94S is used as an inner material, a titanium oxide layer is formed on the surface of the inner material, and a charge separation material is partially supported on the surface of the titanium oxide layer.
By employing such a structure, it is possible for the inner Tixe2x80x94Oxe2x80x94N or Tixe2x80x94Oxe2x80x94S to absorb light in the range from ultraviolet to visible while allowing catalytic reaction by the titanium oxide on the surface and the charge separation material partially supported thereon. Titanium oxide is inexpensive and stable, and an effective catalytic reaction can be realized while preventing recombination of electrons and holes by Pt, Pd, Ni, RuOx (for example, RuO2), NiOx (for example, NiO), SnOx (for example, SnO2), AlxOy (for example, Al2O3), ZnOx (for example, ZnO), and SiOx (for example, SiO2).
According to another aspect of the present invention, there is provided a photocatalytic material comprising an oxide crystal of a metal element M1, the oxide crystal having a photocatalytic function, in which a nitrogen atom or a sulfur atom substitutes for a portion of the oxygen sites of the oxide crystals, is doped at an interstitial site of the crystal lattices of oxide, or is placed at the grain boundary of the polycrystalline body of oxide crystals, and wherein at least one metal element M2 of vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc(Zn), ruthenium (Ru), rhodium (Rh), rhenium (Re), osmium (Os), palladium (Pd), platinum (Pt), iridium (Ir), niobium (Nb), and molybdenum (Mo) substitutes for a portion of the M1 sites of the oxide crystal, is doped at an interstitial site of the crystal lattices of the oxide, or is placed at the grain boundary of the polycrystalline body of the oxide crystals.
Here, it is preferable that the compositional ratio of nitrogen exceeds 0 and is less than 13 in the ratio percent of number of atoms and that the compositional ratio of various metal elements exceeds 0 and is less than 5 in the ratio percent of number of atoms. The compositional ratio of sulfur is similar to that of nitrogen.
In such a photocatalytic material, the absorption edges in the light absorption spectrum are shifted towards a longer wavelength compared to TiO2, Tixe2x80x94Oxe2x80x94N, and Tixe2x80x94Oxe2x80x94S. Therefore, light of a longer wavelength is absorbed and generates a photocatalytic effect. As a result, the efficiency of photocatalytic functions, that is, characteristics such as decomposition of organic matters, decomposition of poisonous gases, purification of water, or the like, can be improved for cases when the sunlight or fluorescent lamp is used as a light source. Moreover, the photocatalytic material enables realization of hydrophilic characteristic and anti-fogging property on the surface not only by irradiation of ultraviolet ray, but also by irradiation of visible light, and maintains such characteristic for a longer period of time.
The cause of this can be considered as follows. The valence band of a semiconductor whose characteristics are dominated by oxygen, O, is affected by doping of nitrogen, N, or sulfur, S. Similarly, conduction band characteristics dominated by Ti are affected by the doping of metals. As a result, one or more new energy levels are created within the band gap (forbidden band) of the oxide such as TiO2, and the effective band gap is narrowed. Consequently, electrons and holes can be produced by absorbing light of lower energy and longer wavelength than in the cases of TiO2, Tixe2x80x94Oxe2x80x94N, and Tixe2x80x94Oxe2x80x94S.
According to another aspect of the present invention, it is preferable that the metal element M1 be formed from any of titanium (Ti), zinc (Zn), and tin (Sn). The oxide of these metal elements M1 functions as a photocatalyst, and the operational light is shifted towards longer wavelength by doping a metal element M2 as described above.
According to another aspect of the present invention, it is preferable that the photocatalytic material is used as an inner material, and that a titanium oxide or a Tixe2x80x94Oxe2x80x94N layer or a Tixe2x80x94Oxe2x80x94S layer which is a titanium oxide containing nitrogen or sulfur is formed as an outer material.
In this manner, by placing Tixe2x80x94Crxe2x80x94Oxe2x80x94N or the like as an inner material, visible light of long wavelength can be effectively absorbed and electrons and holes can be produced there. The electrons and holes migrate to TiO2, Tixe2x80x94Oxe2x80x94N, or Tixe2x80x94Oxe2x80x94S at the surface and superior hydrophilicity, contamination prevention, or decomposition of organic matters can be achieved on the surface. In addition, by placing more stable TiO2, Tixe2x80x94Oxe2x80x94N or Tixe2x80x94Oxe2x80x94S on the front-most surface, the long-term stability of the structure can be improved compared to a structure of simply Tixe2x80x94Crxe2x80x94Oxe2x80x94N or the like.
According to another aspect of the present invention, it is also preferable that the compositional ratios in the outer material and in the inner material gradually change according to the distance from the surface.
The photocatalyst according to this aspect of the present invention basically comprises a titanium compound (Tixe2x80x94Oxe2x80x94N or Tixe2x80x94Oxe2x80x94S) in which a nitrogen atom (N) or a sulfur atom (S) substitutes for a portion of the oxygen sites of a metal oxide such as titanium oxide, is doped at an interstitial site of lattices, or is placed at the grain boundary of polycrystalline body.
Such a metal oxide, for example, Tixe2x80x94Oxe2x80x94N or Tixe2x80x94Oxe2x80x94S in which nitrogen or sulfur is contained in titanium oxide crystals, demonstrates photocatalytic effects when exposed to light in the visible and ultraviolet ranges.
Furthermore, by further doping (co-doping) a metal as described above to Tixe2x80x94Oxe2x80x94N or Tixe2x80x94Oxe2x80x94S, light at even longer wavelengths can also be effectively absorbed.